This invention relates to a method for producing high-purity metallic chromium and, particularly, it relates to a method for producing metallic chromium with a very low concentration level of impurities such as sulfur, nitrogen and oxygen. Such high-purity metallic chromium can be suitably used as a raw material for the electronic industry as well as for the industry of producing corrosion-resistive and heat-resistive alloys (super alloys).
Known methods for producing metallic chromium include the electrolytic method that decomposes Cr.sub.2 (SO.sub.4).sub.3 by electricity and the alumino-thermite reduction method that reduces Cr.sub.2 O.sub.3. However, metallic chromium obtained by any of these known methods contains S, O and N at a relatively high level and, therefore, is not good for manufacturing electronic products.
More specifically, said electrolytic method uses Cr.sub.2 (SO.sub.4).sub.3 as electrolyte and, therefore, the resultant metallic chromium contains S at a relatively high level of concentration between 200 and 300 ppm and contains O at a level between 3,000 and 10,000 ppm and N between 200 and 500 ppm because of the use of aqueous electrolyte.
On the other hand, metallic chromium obtained by the thermite reduction method contains S at a level of concentration as high as between 200 and 400 ppm because of the fact that sulfuric acid is used for production of Cr.sub.2 O.sub.3 to be used as the source material and that almost all the sulfur contained in the source material remains in the resultant metallic chromium. While the O content can be decreased by increasing the rate of reducing agent (aluminum) to be added to the source material, this in turn causes the aluminum to remain in the resultant metallic chromium at a high concentration level. If the rate of the use of aluminum should be reduced, the O concentration level of the obtained metallic chromium becomes inevitably as high as 1,000 to 4,000 ppm. The N concentration level will also be as high as approximately 200 ppm.
Since metallic chromium produced by any of the known methods contains S, O and N at a relatively high concentration level, these impurities should be thoroughly removed from the metallic chromium if it be suitably used for its applications.
The vacuum carbon reduction method and the hydrogen reduction method are among the known methods for degassing metallic chromium. With the vacuum carbon reduction method, carbon powder is added to powdered crude metallic chromium and the mixture is then heated in vacuum to remove the oxygen contained in the metallic chromium after turning it into CO. The hydrogen reduction method is, on the other hand, a method of degassing metallic chromium by heating powdered metallic chromium in an atmosphere of hydrogen and causing the oxygen contained in it to change to H.sub.2 O.
However, any of the above described known methods cannot meet the requirement of manufacturing high-purity metallic chromium which is needed for highly advanced electronic products.
In view of these circumstances, one of the inventors of the present invention has proposed a method for manufacturing high-purity metallic chromium with a very low concentration level of impurities such as S, O and, N as disclosed in Japanese Patent Publication No. 3-79412. The proposed method in fact consists in combining a method of heating in vacuum powder of crude metallic chromium with that of easily sulfidable metals such as Sn, Ni and Cu and the vacuum carbon reduction method or the hydrogen reduction method as described above.
It has been proved that the proposed method is very effective in manufacturing high-purity metallic chromium with a very low concentration level of impurities and, therefore, can be suitably used for various applications including those described above.
However, since the proposed method requires a high degree of vacuum and elevated temperature for heat treatment of crude metallic chromium in vacuum, it inevitably entails a problem of sublimated metallic chromium, which eventually adheres to the heating elements and the lining of furnace to damage the furnace and reduce its heat treatment capacity so that consequently the capability of the furnace to produce high-purity metallic chromium on a stable basis may be significantly adversely affected. There may also arise a problem of contamination of produced metallic chromium by the metallic material of the heating elements of furnace if the heating elements are made of metal. Additionally, there may also be a problem of malfunction of furnace due to prolonged furnace operation involving vacuum and high temperature in an attempt to reduce the concentration level of impurities in the produced metallic chromium as low as possible.